1. Field of the Invention
The present invention relates to an aberration compensation device which compensates for the aberration induced in a light beam irradiated on a storage medium and reflected therefrom, and an optical pickup and information reproducing and/or recording apparatus employing the aberration compensation device. More specifically, the present invention relates to an aberration compensation device which introduces a phase difference to the light beam before the light beam is irradiated on the storage medium.
2. Description of the Prior Art
There is known a method of compensating for aberration induced in a light beam used for reproducing information as described above. In such a method, a liquid crystal panel having electrodes arranged on both faces of the liquid crystal layers is used to compensate for wavefront aberration resulting from, for example, a tilt of the optical axis of the light beam with respect to the information recording surface of the storage medium. This compensation method using the liquid crystal panel utilizes such a phenomenon that the orientation of liquid crystal molecules of in liquid crystal change dependently upon the applied voltage, and compensates for wavefront aberration resulting from the tilt by changing reflective index of the liquid crystal with respect to the light beam passing therethrough. Namely, by changing the voltage applied to each liquid crystal portion to vary the reflective indexes, the optical path lengths of the light beams are differentiated between different liquid crystal portions (i.e., different phase differences are introduced to the respective sectional portions of the light beams). By this,the optical path length to the information recording surface is varied to cancel out wavefront aberration resulting from the tilt between the information recording surface and the optical axis.
On the other hand, there are known various methods of actually differentiating the optical path difference between different liquid crystal portions. In one example, a transparent electrode for applying the drive voltage to the liquid crystal (normally, arranged on one of or both sides of the liquid crystal) is configured as a combination of a plurality of partial transparent electrodes, and the drive voltages applied to the respective partial transparent electrodes are varied to differentiate the phase difference introduced to the light beams by those partial transparent electrodes.
As is conventionally known, when applying the drive voltage to the liquid crystal to change its orientation and thereby to introduce the phase difference to the light beam passing through the liquid crystal, the drive voltage-phase difference characteristic, which shows the relation between the drive voltage and the phase difference introduced by the drive voltage, varies dependently upon ambient temperature. Namely, as shown in the example of FIG. 1, the range of the linear area of the drive voltage-phase difference characteristic varies dependently upon the temperature, and the slope of the characteristic in the linear area also varies dependently upon ambient temperature, Under such circumstance that the linear area of the drive voltage-phase difference characteristic varies dependently upon the temperature, if a certain drive voltage corresponding to an area having nonlinear linear drive voltage-phase difference characteristic is applied at a certain temperature, the phase difference which is obtained at the temperature in the linear area of the drive voltage-phase difference characteristic can not be obtained, and hence an appropriate phase difference can not be introduced to the light beam. Thus, a residual error in the aberration compensation increases, and a tilt margin reduces. In order to avoid the reduction of the tilt margin, in the case of applying drive voltage corresponding to the non-linear area of the drive voltage-phase difference characteristic, separate non-linear circuits for compensating for the non-linearity is provided and controlled in different control methods from temperature to temperature, thereby driving the liquid crystal panel while compensating for such non-linearity. However, providing such separate non-linear circuit to compensate for the drive voltage-phase difference characteristic requires separate non-linearity control for each temperature, which makes the configuration complicated and increases the manufacturing cost.
On the other hand, when the slope of the characteristic within the linear area varies dependently upon ambient temperature, if a constant drive voltage, independent of the temperature, is applied at all temperature to drive the liquid crystal panel in order to simplify the compensation operation, the phase difference actually introduced to the light beam varies dependently upon the temperature variation. This makes the compensation of wavefront aberration unsatisfactory or incomplete, and a so-called compensation error may take place.
It is an object of the present invention to provide an aberration compensation device which has simple configuration and effectively compensates for the aberration when the drive voltage-phase difference characteristic for applying the phase difference to compensate for the aberration has temperature dependency.
It is another object of the present invention to provide an optical pickup, an information reproducing apparatus and an information recording apparatus which employ the above aberration compensation device and which can reproduce and record information accurately with the aid of the compensation device.
According to one aspect of the present invention, there is provided an aberration compensation device including: a phase difference generator for receiving an input signal and for generating a phase difference used for compensating for an aberration induced in a light beam; a temperature detector for detecting a temperature of the phase difference generator; and a corrector for controlling the input signal to be inputted to the phase difference generator based on the temperature detected by the temperature detector thereby to correct a variation of a characteristic of the phase difference generator resulting from a temperature variation.
Therefore, by controlling the input signal correspondingly to the detected temperature, an appropriate phase difference is generated with the variation of the characteristic of the phase difference generator due to the temperature variation being corrected. Therefore, even ambient temperature varies, the phase difference independent from the temperature variation is introduced to the light beam, and hence the aberration may be effectively compensated for.
The input signal may include a drive signal used to generate the phase difference and a reference signal preset for plural temperature values and functioning as a reference for the drive signal, and the corrector may correct the variation of the characteristic by controlling at least one of the reference signal and the drive signal based on the temperature detected by the temperature detector. By this, the variation of the characteristic can be corrected easily and effectively.
The corrector may correct the variation of the characteristic by regarding the input signal when the phase difference generator is at a temperature higher than a preset reference temperature as the input signal lower than the input signal when the phase difference generator is at room temperature. Thus, the temperature dependency of the phase difference can be corrected.
The corrector may correct the variation of the characteristic by controlling the drive signal based on the temperature detected by the temperature detector and a preset drive signal-phase difference characteristic indicating a relation between the drive signal and the phase difference generated by the drive signal. Therefore, the variation of the characteristic can be effectively corrected on the basis of the specific drive signal-phase difference characteristic of the phase difference generator.
The corrector may correct the variation of the characteristic by controlling the input signal based on the temperature detected by the temperature detector and a preset input signal-phase difference characteristic indicating a relation between the input signal and the phase difference generated by the drive signal. Therefore, the variation of the characteristic can be effectively corrected on the basis of the specific input signal-phase difference characteristic of the phase difference generator.
The corrector may generate the reference signal such that the drive signal is applied to the phase difference generator within a range where the input signal-phase difference characteristic is linear. By this, the phase difference generator can be driven linearly to generate appropriate phase difference.
The corrector may include a memory unit for storing a plurality of reference signals preset in correspondence with the temperature values, and the corrector may read out the reference signal corresponding to the temperature detected by the temperature detector from the memory unit to generate the input signal and correct the variation of the characteristic. Thus, the variation of the characteristic can be corrected with controlling the reference signal by using a simple configuration.
In a preferred embodiment, the aberration is induced in the light beam due to a tilt generated between an optical axis of a light beam and a storage medium when the light beam is irradiated on the storage medium. Thus, the aberration induced between the optical axis of the light beam and the storage medium can be reliably compensated for.
According to another aspect of the present invention, there is provided an optical pickup including: the aberration compensation device described above; a light emitter for emitting a light beam to be incident upon the aberration compensation device; a tilt detector for detecting the tilt; and a light receiver for receiving the light beam after the aberration is compensated for and for generating a light reception signal. The corrector may generate the input signal and input the input signal to the phase difference generator, the input signal including the drive signal used to introduce the phase difference to the light beam to compensate for the aberration due to the tilt detected by the tilt detector. Therefore, information can be accurately recorded and reproduced by a simple configuration, with wavefront aberration being compensated for.
According to still another aspect of the present invention, there is provided an information reproduction apparatus including: the optical pickup described above; and a reproduction unit for reproducing information from the storage medium based on the light reception signal. Therefore, information can be accurately reproduced with the aberration due to the tilt between the light beam axis and the storage medium being effectively compensated for.
According to still another aspect of the present invention, there is provided an information recording apparatus including: the optical pickup described above; a reproduction unit for reproducing information from the storage medium based on the light reception signal; and a recording unit for controlling the light beam based on the information reproduced by the reproduction unit and recording information thereby to record the recording information. Therefore, information can be accurately recorded with the aberration due to the tilt between the light beam axis and the storage medium being effectively compensated for.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiment of the invention when read in conjunction with the accompanying drawings briefly described below.